Japanese Laid-Open Patent Publication No. 2010-122336 describes an example of constant current control on a light emitting diode (LED) with a DC-DC converter. Further, as described in WO2004/019148, a sense resistor and an FET may be coupled in series to a light emitting diode string. In this case, the current flowing through the light emitting diode is measured from the voltage generated by the sense resistor to control the ON resistance of the FET based on the measurement. This controls the current flowing through the light emitting diode to be generally constant.
When executing constant current control with a DC-DC converter, the current flowing through the light emitting diode is not directly monitored. Thus, when variations occur between light emitting diodes or elements, the current flowing through the light emitting diode also varies. This results in the necessity to provide an excessive design margin for the current. Further, when monitoring the current that flows through a light emitting diode with a sense resistor, a loss results from the sense resistor.